Predicion of Elastic Properties of Carbon Plastics Obtained by Vacuum Bag Resin Infusion and Direct Prepreg Pressing Technologies

Technical Summary: Prediction of Elastic Properties of Carbon Plastics Obtained by Vacuum Bag Resin Infusion and Direct Prepreg Pressing Technologies

Problem Statement
The development of advanced biomedical structures, particularly energy-storing-and-releasing (ESR) foot prostheses, requires carbon fiber composite materials (CFCMs) with precise stiffness and strength characteristics. These components must support significant loads (up to four times the patient's weight) while maintaining minimal weight and predictable deformation (5–20 mm deflection). A critical challenge lies in selecting the appropriate manufacturing technology and spatial arrangement of components to achieve these specific elastic properties. While vacuum bag resin infusion (VBRI) and direct prepreg pressing (DPP) are widely used, there is a need to compare their resulting mechanical properties and to validate homogenization models for predicting the effective elastic moduli of the resulting orthotropic materials.

Methodology
The study employed a comparative approach involving both experimental testing and analytical prediction:

• Sample Preparation: Five series of CFCM specimens were fabricated using two distinct technologies:

  • VBRI: Utilizing carbon fabrics (T300 twill, T700 unidirectional) and an epoxy resin system (EPR320 + EPH550).
  • DPP: Utilizing pre-impregnated carbon fabrics (YZ-05 T700 twill and unidirectional) pressed in a hydraulic press.
  • The specimens featured varying layup schemes (mixtures of unidirectional and twill fabrics) to achieve a target thickness of approximately 4 mm. Fiber volume fractions were calculated based on density measurements, revealing higher reinforcement levels in DPP specimens (up to 69%) compared to VBRI specimens (approx. 50–52%).

• Experimental Testing:

  • Uniaxial Compression: Performed on an Instron 8801 machine (ASTM D3410) to determine compressive elastic moduli.
  • Three-Point Bending: Performed on a TestSystems UTS-111.2-10-12 machine (ASTM D790) to determine flexural elastic moduli.
  • Tests were conducted on series N1, N3, N5, N7, and N19, with elastic moduli calculated from the linear elastic region of the stress-strain diagrams.

• Analytical Prediction:

  • A two-stage averaging approach based on classical laminate theory was applied.
  • Micromechanical Level: Elastic moduli of individual layers were calculated by averaging fiber and matrix properties using the Voigt (Rule of Mixtures) and Halpin-Tsai models.
  • Macromechanical Level: The properties of the layered structure were averaged using generalized Hooke's law coefficients, accounting for fiber orientation angles and layer thickness.

Key Results

• Manufacturing Influence: Specimens produced via Direct Prepreg Pressing (DPP) consistently exhibited higher elastic moduli and tensile strength compared to those produced via Vacuum Bag Resin Infusion (VBRI). This is attributed to the higher carbon fiber volume fraction achieved in DPP (approx. 63–69%) versus VBRI (approx. 50–52%).
• Bending vs. Compression: A significant discrepancy was observed between loading modes. Compressive elastic moduli were approximately half the values of flexural moduli for the same series, and failure strains were 1.5 to 2.5 times higher in compression.
• Model Accuracy:
  • Bending: The analytical model showed reasonable correlation with experimental bending data for series N1, N3, N5, and N7, though it tended to overestimate values (differences ranging from 5% to 32%). The Halpin-Tsai correction did not significantly alter predictions for high-volume unidirectional layers oriented along the loading axis.
  • Compression: The model demonstrated significant limitations in predicting compressive behavior. Discrepancies between calculated and experimental compressive moduli were substantial (1.5 to 2.3 times higher in calculations).
  • Exception (Series N19): The N19 series, characterized by a high proportion of twill fabric and 45° fiber orientations, showed a unique result where the calculated and experimental compressive moduli nearly coincided (4% difference), though the flexural prediction remained inaccurate (80% difference).

Significance and Conclusions
The study concludes that while sequential averaging models (Voigt and Halpin-Tsai combined with classical laminate theory) can predict the flexural elastic moduli of orthotropic CFCMs with an accuracy of at least 35% for specific layups, they have limited applicability for predicting compressive properties. The significant divergence in compression results suggests that current homogenization models fail to account for heteromodality (different moduli in tension vs. compression) and specific microstructural heterogeneities.

The authors assert that both VBRI and DPP technologies are viable for producing structural elements for foot prostheses, provided the carbon fiber content is sufficient. However, for accurate design of biomedical components, reliance solely on standard homogenization models for compressive stiffness is insufficient. Future improvements in prediction accuracy require models that incorporate stacking sequences, weave types, and distinct tension-compression behaviors, alongside further experimental characterization under uniaxial tension and torsion.